The process of assembling an aircraft requires the use of many fillers, or shims, placed between parts that are to be assembled. The need for shims between assembled parts in aircraft assembly is particularly important given the relatively tight tolerances. For example, tolerances may often require that shims be used when a gap between assembled parts exceeds 0.005 inches. It is not uncommon for a typical commercial aircraft to require over 5,000 shims. In particular, the wing by itself may require over 3,000 shims.
Compounding the problem is the lengthy process required from the time a gap is determined and measured to the time the shim is finally manufactured and installed. Typically, two pieces are brought together as they would be during assembly. A mechanic would then take a series of measurements of the gap between the assembled parts, these measurements to be later used for the manufacture of a shim to fill the gap. Measurements could be taken, for example, using a handheld device with a capacitance gauge to detect the distance between the pieces to be assembled. An example of such a device is the GAPMAN device sold by Capacitec, Inc.
The mechanic would take a sufficient number of gap measurements, for example with the GAPMAN device, to create a grid of X-Y points with corresponding Z measurements (from the measuring device) corresponding to the shim thickness. A mechanic would typically record these XYZ measurements manually in either handwritten form or directly into a computer program, such as a Microsoft Excel spreadsheet. A single shim could require upwards of 200 hand-taken measurements. That number of measurements is multiplied by the total number of shims that are required, which as indicated above, is typically in the thousands.
At the conclusion of the process described above, there will be a significant amount of raw data in the form of XYZ measurements. However, this raw data by itself is not usable for ultimately creating the shim until it is converted into another format that could be understood by the shim manufacturing equipment. Therefore, a next step in the process is to take the gap measurement raw data and convert that data into a format compatible with a solid modeling program, such as Polyworks®.
A second technician in the process, skilled in the use of solid modeling software, would take the converted data and manipulate the resulting solid model as needed. For example, the conversion process from the raw data to the solid model data could result in the creation of outliers or anomalies or other problems in the data that are to be eliminated or otherwise resolved before a shim can be machined.
After the second technician has manipulated the solid model data to get it into shape for machining, a second data conversion is needed to generate the requisite data to manufacture the shim. For example, this third form of data could be in the form of machine code for a computer numerical control (CNC) machine. The CNC code is configured to instruct the machining of a workpiece in accordance with the geometry of the solid model data to finally manufacture the shim.
A third technician, skilled in the use of CNC machines (or other manufacturing methods), would be in charge of creating the machine code and in the setup of the manufacturing equipment and ultimate creation of the shim.
In the end, the above process—the taking of gap measurements, conversion into solid model data, manipulation of the solid model data, creation of machine code, and final manufacturing of the shim—could require more than 8 hours. Thus, the aircraft assembly process would be put on hold to accommodate the time required to create the needed shims, often requiring the assembly to be stopped until the following day.